Gas feedstocks, such as refinery-off gases (ROG) and natural gas (NG), are important sources for the production of light valuable commodities, including H2 and CH4, which are used to feed most catalytic processes in the petrochemical industry. Moreover, ROG and NG are prime sources for the isolation of polymer/chemical-grade light olefins and other paraffins, such as ethane, propane and n-butane (C2+).
Accordingly, the upgrading of ROG and NG, i.e., the elimination of various undesirable gas components, to valuable commodities, such as H2, CH4 and polymer-grade C2H4 and C3H6, is of prime importance and remains a major challenge facing oil and gas engineers/technologists in gas plants. Practically, ROG are collected from a variety of units in oil refineries, and the associated quality/quantity depends on the crude oil composition and the type of refining/separation/purification processes being implemented. Essentially, the ROG stream is mainly generated in the cracking units, where the long-chain hydrocarbon products are cracked to produce shorter molecular chains or lighter hydrocarbon components. Thus, the ROG are composed of various feedstocks and, if recovered, are of economic value as a prospective added revenue for refineries and petrochemical complexes. Namely, recovered H2 supplies hydrocracking processes for the production of valuable olefins and additional gasoline fractions from heavy fractions, whereas CH4 and C2+ are used as feedstocks in catalytic reforming and polyolefin production. Because the compositions of ROG and NG are relatively complex (Tables 1 and 2), the recovery of the valuable products with high purity and the desired specifications can be intricate.
TABLE S1Typical compositionof refinery-off gas (ROG)Components of ROGTypical ROG CompositionH2 ~5-35%  CO0.1-0.5%N2 3-10%CO20.1-0.5%CH430-50%C2H2 1-15 ppm (vol)Ethylene 5-20%Ethane15-25%Methyl acetylene & propadiene60-80 ppm (vol)Propane1-5%Propylene1-5%Butadiene  0-0.1%Butylene0.1-0.3%Butanes0.5-1%  C5+0.2-1.5%
TABLE 2Typical composition of natural gasTypical RawNatural Gas Natural Gas Natural Gas PipelineComponentsCompositionSpecificationsCH4~70-90%>96%CO2Up to 10% <2%Total inerts (N2, CO2)Up to 50% <4%H2SUp to 20% <4 ppmH2OSaturated<120 ppmC2+ hydrocarbonsUp to 20%950-1,050 Btu/scfDew point <−20° C.
For instance, the H2 and CH4 contained in the ROG must be separated from C2+ and H2S, whereas C3H6 must be H2S free to be used as a feedstock for the production of polymers. Clearly, the presence of acid gases, such as H2S, makes ROG and NG upgrading schemes complex and multistage operations, particularly with regard to the poisoning of the adsorbent/catalysts used in ROG processing.
Various approaches have been deployed to tackle these challenging separations, including membrane and/or adsorbent technologies. ROG and NG upgrades are mainly accomplished using energy-intensive separations, such as distillation and chemical-based absorption. Several recent studies have suggested the potential to reduce the high-energy penalty via deploying a new generation of advanced physical adsorbents. In this context, porous solid adsorbents have great potential for gas/vapor separations, driven either by (i) equilibrium, (ii) kinetics processes, (iii) a combination thereof and/or molecular sieving. The successful deployment of any of the aforementioned separation mechanisms is directly correlated to the ability to fine-tune the pore aperture size and/or functionality in a selected porous material.
Metal-organic frameworks (MOFs) have received considerable attention as adsorbents or membranes for gas separation applications. This relatively new family of hybrid porous solids has been explored for the selective adsorption of hydrocarbon mixtures based on different separation mechanisms. Essentially, equilibrium-based processes are the conventional routes for hydrocarbon separation using MOFs. Recently, kinetics and full molecular sieving based-separations of hydrocarbons have been reported using MOF adsorbents because of their unique tunable structural/chemical features with respect to the other classes of porous materials.
Although a wide assortment of MOF modulations for targeted separation have already been proposed, combined experimental and theoretical studies focusing on this family of materials for the selective ad-sorption of hydrocarbons (vs CH4 and H2) are scarce. Furthermore, only a very limited number of MOFs have been shown to retain their structural integrity and stability upon H2S adsorption. The highly challenging pre-treatment (prior to the olefin-paraffin separation) of ROG (or NG) using MOFs as adsorbents/membranes for the production of high-purity H2, CH4 and polymer-grade C2+ streams has not been considered previously.